1. Field of the Invention
The present invention relates to an air-fuel ratio control system and method and an engine control unit for an internal combustion engine, for controlling the air-fuel ratio of a mixture supplied to a plurality of cylinders of the engine on a cylinder-by-cylinder basis.
2. Description of the Related Art
Recently, it is demanded of internal combustion engines due to social requirements that the engines have excellent exhaust emission characteristics, that is, an excellent emission reduction rate of the catalyst. On the other hand, internal combustion engines having a plurality of cylinders can suffer variation in air-fuel ratio between the cylinders to which a mixture is supplied, due to the malfunction of an EGR system, an evaporative fuel processing system, or injectors. In such a case, there is a fear of the emission reduction rate of the catalyst being lowered. Therefore, to overcome the problem, as an air-fuel ratio control system for an internal combustion engine, which corrects (absorbs) variation in air-fuel ratio between cylinders, there has been conventionally proposed one to which is applied an observer based on the optimal control theory (see e.g. Publication of Japanese Pat. No. 3296472, pages 18-23, FIGS. 35 and 36). This air-fuel ratio control system is comprised of a LAF sensor disposed in the collecting section of an exhaust pipe, for detecting the air-fuel ratio of exhaust gases, a control unit to which a detection signal (indicative of the detected air-fuel ratio) from the LAF sensor is input, and injectors disposed in the intake manifold of the exhaust pipe for the respective cylinders and connected to the control unit.
In this control unit, the variation in the air-fuel ratio of the mixture supplied to each cylinder is corrected by calculating a cylinder-by-cylinder fuel injection amount #nTout (n=1 to 4) as the amount of fuel to be injected into each cylinder, based on the detected air-fuel ratio output from the LAF sensor, using the observer and by PID control, as described below.
That is, the control unit calculates the basic fuel injection amount Tim depending on the operating conditions of the engine, and multiplies the basic fuel injection amount by various correction coefficients to calculate the output fuel injection amount Tout. Then, as described below, the observer calculates a cylinder-by-cylinder estimated air-fuel ratio #nA/F, and a cylinder-by-cylinder estimated feedback correction coefficient #nKLAF is determined by PID control based on the estimated cylinder-by-cylinder air-fuel ratio #nA/F. The cylinder-by-cylinder fuel injection amount #nTout is calculated by multiplying the output fuel injection amount Tout by the cylinder-by-cylinder feedback correction coefficient #nKLAF.
The cylinder-by-cylinder estimated air-fuel ratio #nA/F is calculated by the observer based on the optimal control theory. More specifically, by using a model of a discrete-time system representative of the relationship between a cylinder-by-cylinder fuel-air ratio and a fuel-air ratio detected at the collecting section (where the LAF sensor is disposed), the cylinder-by-cylinder air-fuel ratio #nA/F is calculated. Further, in the PID control, a value obtained by dividing the fuel-air ratio detected at the collecting section, i.e. the detected air-fuel ratio KACT, by the average value of the respective preceding values of the feedback correction coefficients #nKLAF is set to a target value, and the cylinder-by-cylinder feedback correction coefficient #nKLAF is calculated such that the difference between the target value and the cylinder-by-cylinder estimated air-fuel ratio #nA/F calculated by the observer converges to a value of 0.
Recently, aside from the above-mentioned demand of excellent exhaust emission characteristics, internal combustion engines are demanded of higher power output and higher torque. To meet the demand, there is employed the technique of reducing the exhaust resistance by configuring the layout of the exhaust system into a complicated shape (in which exhaust passages from the cylinders are progressively combined in the exhaust manifold such that four passages, for example, are combined into two passages, and the two passages are then combined into one passage). However, when the conventional air-fuel ratio control system is applied to internal combustion engines having such an exhaust system layout, the observer can be no longer applicable based on the conventional optimal control theory, and therefore, the variation in air-fuel ratio between the cylinders cannot be properly corrected, which can lead to a lowered emission reduction rate of the catalyst. This is because according to the conventional optimal control theory, modeling errors and changes in the dynamic characteristics of a model are not considered in the simulation model and the optimal control theory itself, which makes the observer small in margin of stability and low in robustness. Therefore, the air-fuel ratio control system does not have a sufficient stability against changes in the contributions of exhaust gases from the individual cylinders to the detected air-fuel ratio of the LAF sensor caused by attachment of fuel, etc., changes in the response of the LAF sensor, and the aging of the same.
It is an object of the present invention to provide an air-fuel ratio control system and method and an engine control unit for an internal combustion engine, which are capable of appropriately and promptly correcting variation in air-fuel ratio between a plurality of cylinders and realizing a very robust air-fuel ratio control.
To attain the above object, in a first aspect of the present invention, there is provided an air-fuel ratio control system for an internal combustion engine including a plurality of cylinders and an exhaust passage through which exhaust gases from the cylinders flow, the air fuel ratio control system controlling an amount of fuel to be supplied to each of the cylinders, on a cylinder-by-cylinder basis, to thereby control an air fuel ratio of a mixture supplied to each cylinder.
The air-fuel ratio control system according to the first aspect of the present invention is characterized by comprising:
fuel amount-determining means for determining an amount of fuel to be supplied to each cylinder;
correction parameter-determining means for determining a correction parameter for correcting the amount of fuel to be supplied to each cylinder;
first fuel amount-correcting means for correcting the determined amount of fuel to be supplied to each cylinder, according to the determined correction parameter;
air-fuel ratio parameter-detecting means for detecting an air-fuel ratio parameter indicative of an air-fuel ratio of the exhaust gases flowing through the exhaust passage;
variation parameter-calculating means for calculating a variation parameter indicative of a variation in air-fuel ratio between the plurality of parameters, on a cylinder-by-cylinder basis, based on a model parameter of a model formed by modeling each cylinder and having an input of the correction parameter and an output of the air-fuel ratio parameter;
identification means for identifying the model parameter of the model based on the determined correction parameter and the detected air-fuel ratio parameter; and
second fuel amount-correcting means for further correcting the amount of fuel to be supplied to the plurality of cylinders on a cylinder-by-cylinder basis such that the variation parameter calculated on a cylinder-by-cylinder basis converges to a predetermined target value.
With the arrangement of the air-fuel ratio control system according to the first aspect of the present invention, an amount of fuel to be supplied to each cylinder is determined by the fuel amount-determining means, and corrected according to a correction parameter by the first fuel amount-correcting means. Further, a variation parameter indicative of a variation in air-fuel ratio between the plurality of parameters is calculated by the variation parameter-calculating means, on a cylinder-by-cylinder basis, based on a model parameter of a model formed by modeling each cylinder and having an input of the correction parameter and an output of the air-fuel ratio parameter, and the amount of fuel to be supplied to each cylinder is further corrected by the second fuel amount-correcting means such that the variation parameter calculated on a cylinder-by-cylinder basis converges to a predetermined target value. That is, the amount of fuel to be supplied to the engine is corrected such that variation in air-fuel ratio between the cylinders is corrected. Further, the model parameter of the model is identified based on the determined correction parameter and the detected air-fuel ratio parameter by the identification means. As described above, the variation parameter for correcting the amount of fuel to be supplied to each cylinder is calculated based on the model parameter identified based on the correction parameter and the air-fuel ratio parameter, and hence, by using e.g. an onboard identifier for the identification means, it is possible to calculate the variation parameter based on the model parameter identified in real time. Therefore, differently from the prior art, even when the dynamic characteristics of a controlled object are changed due to changes in respective contributions of the individual cylinders (exhaust gasses therefrom) to the detected air-fuel ratio, which are caused by attachment of fuel in the cylinders, changes in the response of the air-fuel ratio parameter-detecting means, and aging of the air-fuel ratio parameter-detecting means, it is possible to correct the variation in air-fuel ratio between the cylinders while causing the changes in the dynamic characteristics of the controlled object to be reflected in the model. As a result, even when the air-fuel ratio control system is applied to an internal combustion engine having a complicated exhaust system layout, it is possible to realize a highly robust air-fuel ratio control, and thereby maintain an excellent emission reduction rate of the catalyst.
Preferably, the air-fuel ratio control system further comprises target value-setting means for setting an average value of the variation parameter to the predetermined target value.
With the arrangement of this preferred embodiment, an average value of the variation parameter is set to the predetermined target value by the target value-setting means, and therefore, the amount of fuel to be supplied to each cylinder is corrected by the second fuel amount-correcting means such that the variation parameter converges to the average value without being diverged. Thus, the amount of fuel to be supplied to each cylinder is corrected such that the variation parameter converges to the average value thereof, which makes it possible to correct the variation in air-fuel ratio between the cylinders. As a result, it is possible to correct the variation in air-fuel ratio between the cylinders while avoiding interfering with correction of the amount of fuel by the first fuel amount-correcting means, i.e. the air-fuel ratio control. For the same reason, even when another type of feedback control or feedforward control is executed in parallel in the air-fuel ratio control, it is possible to correct variation in air-fuel ratio between a plurality of cylinders while avoiding interference with such control.
Preferably, the correction parameter-determining means determines the correction parameter such that the air-fuel ratio parameter is caused to converge to a predetermined target air-fuel ratio value.
With the arrangement of this preferred embodiment, the correction parameter is determined by the correction parameter-determining means such that the air-fuel ratio parameter is caused to converge to a predetermined target air-fuel ratio value, and the amount of fuel to be supplied to each cylinder is corrected by the first fuel amount-correcting means according to the correction parameter. In other words, the correction parameter and the variation parameter are calculated separately, and at the same time, corrections of the amount of fuel responsive to the two parameters are separately executed by the different types of fuel amount-correcting means, respectively. This makes it possible to carry out two types of air-fuel ratio control, i.e. air-fuel ratio control for correcting variation in air-fuel ratio between the cylinders, and air-fuel ratio control for causing the air-fuel ratio of exhaust gases to converge to a predetermined target value, without causing interference therebetween. This makes it possible to control the air-fuel ratio of exhaust gases to the predetermined target value while correcting variation in air-fuel ration between the cylinders, and thereby enhance the emission reduction rate of the catalyst.
Preferably, the model parameter is of a model formed by modeling one of the plurality of cylinders, the correction parameter-determining means determining the correction parameter based on the model parameter, and the first fuel amount-correcting means corrects the mount of fuel to be supplied to all of the plurality of cylinders according to the determined correction parameter.
With the arrangement of this preferred embodiment, the correction parameter is determined by the correction parameter-determining means based on the model parameter of a model formed by modeling one of the plurality of cylinders, and the first fuel amount-correcting means corrects the amount of fuel to be supplied to all of the cylinders according to the determined correction parameter. This makes it possible to correct the amount of fuel to be supplied to all the cylinders without interfering with correction of the amount of fuel to be made for correction of variation in air-fuel ratio between the cylinders. Further, as described above, this model parameter is identified by the identification means, and therefore, by using e.g. an onboard identifier as the identification means, the correction parameter can be calculated based on the model parameter identified in real time. Therefore, even when the dynamic characteristics of a controlled object are changed due to changes in respective contributions of the individual cylinders (exhaust gases therefrom) to the detected air-fuel ratio, which are caused by attachment of fuel in the cylinders, changes in the response of the air-fuel ratio parameter-detecting means, and aging of the air-fuel ratio parameter-detecting means, it is possible to correct the amount of fuel to be supplied to all the cylinders while causing the changes in the dynamic characteristics of the controlled object to be reflected in the model, whereby the robustness of air-fuel ratio control can be further enhanced.
Preferably, the second fuel amount-correcting means executes correction of the amount of fuel, based on one of an I-PD control algorithm and an IP-D control algorithm.
With the arrangement of this preferred embodiment, the amount of fuel is corrected based on an I-PD control algorithm or an IP-D control algorithm by the second fuel amount-correcting means. This makes it possible to calculate the correction amount such that the correction of the amount of fuel does not overshoot the target value. This makes it possible to correct variation in air-fuel ratio between the cylinders while preventing the behavior of the air-fuel ratio of each cylinder from becoming oscillatory, thereby enhancing stability of the air-fuel ratio control.
Preferably, the second fuel amount-correcting means executes correction of the amount of fuel, based on a response-specified control algorithm.
With the arrangement of this preferred embodiment, the amount of fuel is corrected by the second fuel amount-correcting means based on the response-specified control algorithm. This makes it possible to calculate the correction amount such that the correction of the amount of fuel does not overshoot the target value and that the variation in air-fuel ratio between the cylinders converges in a specified converging behavior. This makes it possible to correct the variation in air-fuel ratio between the cylinders while preventing the behavior of the air-fuel ratio of each cylinder from becoming oscillatory, thereby enhancing the stability of the air-fuel ratio control.
To attain the above object, in a second aspect of the present invention, there is provided an air-fuel ratio control system for an internal combustion engine including a plurality of cylinders, the air fuel ratio control system controlling an amount of fuel to be supplied to each of the cylinders, on a cylinder-by-cylinder basis, to thereby control an air fuel ratio of a mixture supplied to each cylinder.
The air-fuel ratio control system according to the second aspect of the present invention is characterized by comprising:
first operating condition parameter-detecting means for detecting a first operating condition parameter indicative of an operating condition of the engine;
fuel amount-determining means for determining an amount of fuel to be supplied to each cylinder;
variation correction coefficient-calculating means for calculating a variation correction coefficient for correcting variation in air-fuel ratio between the plurality of cylinders, on a cylinder-by-cylinder basis;
learned value-calculating means for calculating a learned value of the variation correction coefficient, on a cylinder-by-cylinder basis, according to the calculated variation correction coefficient and the detected first operating condition parameter; and
fuel amount-correcting means for correcting the determined amount of fuel to be supplied to each cylinder, according to the calculated learned value of the variation correction coefficient and the calculated variation correction coefficient.
With the arrangement of the air-fuel ratio control system according to the second aspect of the present invention, a first operating condition parameter indicative of an operating condition of the engine is detected by the first operating condition parameter-detecting means, and an amount of fuel to be supplied to each cylinder is determined by the fuel amount-determining means. A variation correction coefficient for correcting variation in air-fuel ratio between the plurality of cylinders is calculated on a cylinder-by-cylinder basis by the variation correction coefficient-calculating means. Further, a learned value of the variation correction coefficient is calculated according to the calculated variation correction coefficient and the detected first operating condition parameter by the learned value-calculating means, and the amount of fuel to be supplied to each cylinder is corrected according to the calculated learned value of the variation correction coefficient and the calculated variation correction coefficient by the fuel amount-correcting means. As described above, the amount of fuel to be supplied to each cylinder is corrected according to the learned value of the variation correction coefficient calculated according to the first operating condition parameter. Therefore, even when the state of variation in air-fuel ratio between the cylinders is changed due to a change in the operating condition of the engine, it is possible to correct the amount of fuel to be supplied to each cylinder, in response thereto. This makes it possible to control the air-fuel ratio, even when the engine is in a transient operating condition, while compensating for a change in the state of variation in air-fuel ratio between the cylinders, whereby it is possible to maintain an excellent exhaust emission reduction rate.
Preferably, the learned value-calculating means calculates the learned value of the variation correction coefficient, by a regression equation using the leaned value as a dependent variable and at the same time using the first operating condition parameter as an independent variable, and calculates a regression coefficient and a constant term of the regression equation by a sequential least-squares method.
With the arrangement of this preferred embodiment, the learned value-calculating means calculates the learned value of the variation correction coefficient, by a regression equation using the leaned value as a dependent variable and at the same time using the first operating condition parameter as an independent variable, and calculates a regression coefficient and a constant term of the regression equation by a sequential least-squares method. Thus, the regression coefficient and the constant term of the regression equation used for calculation of the learned value are calculated by the sequential least-squares method. This makes it possible to calculate the learned value of the variation correction coefficient such that the error between the learned value and the variation correction coefficient is reduced, whereby the accuracy of calculation of the learned value can be enhanced, and even when the operating condition of the engine changes, i.e. the dynamic characteristics of the controlled object change, it is possible to more appropriately correct the variation in air-fuel ratio between the cylinders while causing the change to be reflected in the calculation. Further, due to necessity of calculation of learned values for the plurality of cylinders, respectively, when the least-squares method, for example, is employed as the method of calculating the learned value, it is necessary to carry out inverse matrix calculation and store a large number of data. In the case of this preferred embodiment, however, since the sequential least-squares method is employed, it is possible to calculate the learned values using data sequentially calculated, without executing the inverse matrix calculation and storage of a large number of data. This can reduce the computing time of the learned value. As a result, it is possible to obtain the advantageous effects described above, using a computer having a relatively low computing power, such as a vehicle-mounted computer.
Preferably, the air-fuel ratio control system further comprises second operating condition parameter-detecting means for detecting a second operating condition parameter indicative of an operating condition of the engine, and when the detected second operating condition parameter is not within a predetermined range, the learned value-calculating means calculates the learned value of the variation correction coefficient, on a cylinder-by-cylinder basis, according to a value of the variation correction coefficient calculated when the detected second operating condition parameter was within the predetermined range.
In general, when the engine is in an unstable operating condition, such as a very high load operating condition, the calculation of a parameter indicative of variation in air-fuel ratio, such as the variation correction coefficient used in the present invention, is affected by the unstable operating condition which can vary the result of the calculation. Also when the engine is a very low-load operating condition, the accuracy of the calculation can be lowered e.g. due to lowered detection accuracy of a sensor that detects the air-fuel ratio. Further, when the engine is in a high engine speed operation, the frequency of the air-fuel ratio caused by variation in air-fuel ratio becomes high, which can lower the accuracy of calculation of a parameter indicative of the state of the variation. Thus, there is a fear of being incapable of obtaining an appropriately calculated value of the variation correction coefficient. In the air-fuel ratio control system according to the preferred embodiment, however, when the detected second operating condition parameter is not within a predetermined range, the learned value of the variation correction coefficient is calculated using a value of the variation correction coefficient calculated when the detected second operating condition parameter was within the predetermined range. Therefore, even when the engine is in an unstable operating condition, by properly setting the predetermined range, the learned value can be calculated properly according to the variation correction coefficient calculated when the engine was in a stable operating condition. As a result, it is possible to properly correct the variation in air-fuel ratio between the cylinders while avoiding adverse influence of the unstable operating condition of the engine on the calculation, enhance the stability of the air-fuel ratio control, and maintain an excellent emission reduction rate of the catalyst.
Preferably, the air-fuel ratio control system further comprises operating environment parameter-detecting means for detecting an operating environment parameter indicative of an operating environment of the engine, and when the detected operating environment parameter is not within a predetermined range, the learned value-calculating means calculates the learned value of the variation correction coefficient, on a cylinder-by-cylinder basis, according to a value of the variation correction coefficient calculated when the detected operating environment parameter was within the predetermined range.
In general, when the operating environment of the engine is in an extreme condition, such as a very low outside air temperature, which can cause an unstable operating condition of the engine, there is a fear that the resulting unstable operating condition of the engine causes variation in the result of calculation of a parameter indicative of variation in air-fuel ratio between the cylinders, such as the variation correction coefficient of the present invention, so that there is a fear of being incapable of obtaining an appropriately calculated value of the parameter. In the air-fuel ratio control system according to the preferred embodiment, however, when the operating environment parameter is not within the predetermined range, the learned value of the variation correction coefficient is calculated according to a value of the variation correction coefficient calculated when the operating environment parameter was within the predetermined range. Therefore, by properly setting the predetermined range, even when the engine is in an operating environment, causing an unstable operating condition of the engine, the learned value can be calculated properly according to the value of the variation correction coefficient calculated when the engine was in an operating environment in which the operation of the engine was stable. As a result, it is possible to properly correct the variation in air-fuel ratio between the cylinders while avoiding adverse influence of the operating environment on the calculation, enhance the stability of the air-fuel ratio control, and maintain an excellent emission reduction rate of the catalyst.
To attain the above object, in a third aspect of the present invention, there is provided an air-fuel ratio control method for an internal combustion engine including a plurality of cylinders and an exhaust passage through which exhaust gases from the cylinders flow, the air fuel ratio control method including controlling an amount of fuel to be supplied to each of the cylinders, on a cylinder-by-cylinder basis, to thereby control an air fuel ratio of a mixture supplied to each cylinder.
The air-fuel ratio control method according to the third aspect of the present invention is characterized by comprising:
a fuel amount-determining step of determining an amount of fuel to be supplied to each cylinder;
a correction parameter-determining step of determining a correction parameter for correcting the amount of fuel to be supplied to each cylinder;
a first fuel amount-correcting step of correcting the determined amount of fuel to be supplied to each cylinder, according to the determined correction parameter;
an air-fuel ratio parameter-detecting step of detecting an air-fuel ratio parameter indicative of an air-fuel ratio of the exhaust gases flowing through the exhaust passage;
a variation parameter-calculating step of calculating a variation parameter indicative of a variation in air-fuel ratio between the plurality of parameters, on a cylinder-by-cylinder basis, based on a model parameter of a model formed by modeling each cylinder and having an input of the correction parameter and an output of the air-fuel ratio parameter;
an identification step of identifying the model parameter of the model based on the determined correction parameter and the detected air-fuel ratio parameter; and
a second fuel amount-correcting step of further correcting the amount of fuel to be supplied to the plurality of cylinders on a cylinder-by-cylinder basis such that the variation parameter calculated on a cylinder-by-cylinder basis converges to a predetermined target value.
With the arrangement of the air-fuel ratio control method according to the third aspect of the present invention, it is possible to obtain the same advantageous effects as provided by the first aspect of the present invention.
Preferably, the air-fuel ratio control method further comprises a target value-setting step of setting an average value of the variation parameter to the predetermined target value.
Preferably, the correction parameter-determining step includes determining the correction parameter such that the air-fuel ratio parameter is caused to converge to a predetermined target air-fuel ratio value.
Preferably, the model parameter is of a model formed by modeling one of the plurality of cylinders, the correction parameter-determining step including determining the correction parameter based on the model parameter, and the first fuel amount-correcting step includes correcting the mount of fuel to be supplied to all of the plurality of cylinders according to the determined correction parameter.
Preferably, the second fuel amount-correcting step includes executing correction of the amount of fuel, based on one of an I-PD control algorithm and an IP-D control algorithm.
Preferably, the second fuel amount-correcting step executes correction of the amount of fuel, based on a response-specified control algorithm.
With the arrangements of these preferred embodiments, it is possible to obtain the same advantageous effects as provided by the corresponding preferred embodiments of the first aspect of the present invention.
To attain the above object, in a fourth aspect of the present invention, there is provided an air-fuel ratio control method for an internal combustion engine including a plurality of cylinders, the air fuel ratio control method including controlling an amount of fuel to be supplied to each of the cylinders, on a cylinder-by-cylinder basis, to thereby control an air fuel ratio of a mixture supplied to each cylinder.
The air-fuel ratio control method according to the fourth aspect of the present invention is characterized by comprising:
a first operating condition parameter-detecting step of detecting a first operating condition parameter indicative of an operating condition of the engine;
a fuel amount-determining step of determining an amount of fuel to be supplied to each cylinder;
a variation correction coefficient-calculating step of calculating a variation correction coefficient for correcting variation in air-fuel ratio between the plurality of cylinders, on a cylinder-by-cylinder basis;
a learned value-calculating step of calculating a learned value of the variation correction coefficient, on a cylinder-by-cylinder basis, according to the calculated variation correction coefficient and the detected first operating condition parameter; and
a fuel amount-correcting step of correcting the determined amount of fuel to be supplied to each cylinder, according to the calculated learned value of the variation correction coefficient and the calculated variation correction coefficient.
With the arrangement of the air-fuel ratio control method according to the fourth aspect of the present invention, it is possible to obtain the same advantageous effects as provided by the second aspect of the present invention.
Preferably, the learned value-calculating step includes calculating the learned value of the variation correction coefficient, by a regression equation using the leaned value as a dependent variable and at the same time using the first operating condition parameter as an independent variable, and calculating a regression coefficient and a constant term of the regression equation by a sequential least-squares method.
Preferably, the air-fuel ratio control method further comprises a second operating condition parameter-detecting step of detecting a second operating condition parameter indicative of an operating condition of the engine, and the learned value-calculating step includes calculating, when the detected second operating condition parameter is not within a predetermined range, the learned value of the variation correction coefficient on a cylinder-by-cylinder basis according to a value of the variation correction coefficient calculated when the detected second operating condition parameter was within the predetermined range.
Preferably, the air-fuel ratio control method further comprises an operating environment parameter-detecting step of detecting an operating environment parameter indicative of an operating environment of the engine, and the learned value-calculating step includes calculating, when the detected operating environment parameter is not within a predetermined range, the learned value of the variation correction coefficient on a cylinder-by-cylinder basis according to a value of the variation correction coefficient calculated when the detected operating environment parameter was within the predetermined range.
With the arrangements of these preferred embodiments, it is possible to obtain the same advantageous effects as provided by the corresponding preferred embodiments of the second aspect of the present invention.
To attain the above object, in a fifth aspect of the present invention, there is provided an engine control unit including a control program for causing a computer to perform an air-fuel ratio control process for an internal combustion engine including a plurality of cylinders and an exhaust passage through which exhaust gases from the cylinders flow, the air fuel ratio control process including controlling-an amount of fuel to be supplied to each of the cylinders, on a cylinder-by-cylinder basis, to thereby control an air fuel ratio of a mixture supplied to each cylinder.
The engine control unit according to the fifth aspect of the present invention is characterized in that the program causes the computer to determine an amount of fuel to be supplied to each cylinder, determine a correction parameter for correcting the amount of fuel to be supplied to each cylinder, correct the determined amount of fuel to be supplied to each cylinder, according to the determined correction parameter, detect an air-fuel ratio parameter indicative of an air-fuel ratio of the exhaust gases flowing through the exhaust passage, calculate a variation parameter indicative of a variation in air-fuel ratio between the plurality of parameters, on a cylinder-by-cylinder basis, based on a model parameter of a model formed by modeling each cylinder and having an input of the correction parameter and an output of the air-fuel ratio parameter, identify the model parameter of the model according to the determined correction parameter and the detected air-fuel ratio parameter, and further correct the amount of fuel to be supplied to the plurality of cylinders on a cylinder-by-cylinder basis such that the variation parameter calculated on a cylinder-by-cylinder basis converges to a predetermined target value.
With the arrangement of the engine control unit according to the fifth aspect of the present invention, it is possible to obtain the same advantageous effects as provided by the first aspect of the present invention.
Preferably, the program causes the computer to set an average value of the variation parameter to the predetermined target value.
Preferably, the program causes the computer to determine the correction parameter such that the air-fuel ratio parameter is caused to converge to a predetermined target air-fuel ratio value.
Preferably, the model parameter is of a model formed by modeling one of the plurality of cylinders, the program causing the computer to determine the correction parameter based on the model parameter, and when the program causes the computer to correct the amount of fuel to be supplied to each cylinder, the program causes the computer to correct the mount of fuel to be supplied to all of the plurality of cylinders according to the determined correction parameter.
Preferably, when the program causes the computer to further correct the amount of fuel to be supplied to the plurality of cylinders, on a cylinder-by-cylinder basis, the program causes the computer to correct the amount of fuel, based on one of an I-PD control algorithm and an IP-D control algorithm.
Preferably, when the program causes the computer to further correct the amount of fuel to be supplied to the plurality of cylinders, on a cylinder-by-cylinder basis, the program causes the computer to correct the amount of fuel, based on a response-specified control algorithm.
With the arrangements of these preferred embodiments, it is possible to obtain the same advantageous effects as provided by the corresponding preferred embodiments of the first aspect of the present invention.
To attain the above object, in a sixth aspect of the present invention, there is provided an engine control unit including a control program for causing a computer to perform an air-fuel ratio control process for an internal combustion engine including a plurality of cylinders, the air fuel ratio control process -including controlling an amount of fuel to be supplied to each of the cylinders, on a cylinder-by-cylinder basis, to thereby control an air fuel ratio of a mixture supplied to each cylinder.
The engine control unit according to the sixth aspect of the invention is characterized in that the program causes the computer to detect a first operating condition parameter indicative of an operating condition of the engine, determine an amount of fuel to be supplied to each cylinder, calculate a variation correction coefficient for correcting variation in air-fuel ratio between the plurality of cylinders, on a cylinder-by-cylinder basis, calculate a learned value of the variation correction coefficient, on a cylinder-by-cylinder basis, according to the calculated variation correction coefficient and the detected first operating condition parameter, and correct the determined amount of fuel to be supplied to each cylinder, according to the calculated learned value of the variation correction coefficient and the calculated variation correction coefficient.
With the arrangement of the engine control unit according to the sixth aspect of the present invention, it is possible to obtain the same advantageous effects as provided by the second aspect of the present invention.
Preferably, the program causes the computer to calculate the learned value of the variation correction coefficient, by a regression equation using the leaned value as a dependent variable and at the same time using the first operating condition parameter as an independent variable, and calculate a regression coefficient and a constant term of the regression equation by a sequential least-squares method.
Preferably, the program causes the computer to detect a second operating condition parameter indicative of an operating condition of the engine, and when the detected second operating condition parameter is not within a predetermined range, the program causes the computer to calculate the learned value of the variation correction coefficient on a cylinder-by-cylinder basis according to a value of the variation correction coefficient calculated when the detected second operating condition parameter was within the predetermined range.
Preferably, the program causes the computer to detect an operating environment parameter indicative of an operating environment of the engine, and when the detected operating environment parameter is not within a predetermined range, the program causes the computer to calculate the learned value of the variation correction coefficient on a cylinder-by-cylinder basis according to a value of the variation correction coefficient calculated when the detected operating environment parameter was within the predetermined range.
With the arrangements of these preferred embodiments, it is possible to obtain the same advantageous effects as provided by the corresponding preferred embodiments of the second aspect of the present invention.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.